Look-up table using multi-level decode

ABSTRACT

A look-up table circuit implemented with MOS transistors that uses combinational logic to generate signals that enable the transistors. A circuit using 16 inputs and 4 select lines is disclosed. Two of the select lines are used as inputs to combinational logic including four NOR gates to generate enable signals for transistors in a third stage of the circuit. This produces a reduction in the propagation delay of a signal from the input to the output of the look-up table circuit.

This application is a continuation of application Ser. No. 08/591,121, filed Jan. 25, 1996; which was a continuation of application Ser. No. 08/335,628, filed Nov. 8, 1994, now abandoned; which was a continuation of application Ser. No. 08/076,712, filed Jun. 11, 1993, now U.S. Pat. No. 5,438,295.

FIELD OF THE INVENTION

This invention relates generally to electronic look-up table circuits and specifically to a look-up table circuit that uses combinational logic to generate internal enable signals in a programmable logic array application.

BACKGROUND OF THE INVENTION

Look-up table circuits are well known in the art for selecting one of several signal inputs and passing the selected signal input to the output of the circuit. Look-up tables are widely used in applications such as communications, digital computing, control systems, etc.

FIG. 1 shows a block diagram of a look-up table 100 having signal inputs 102, select lines 104 and output 106. In operation, signal inputs 102 are applied with electrical signals in the form of, e.g., digital data, represented by high and low voltages corresponding, respectively, to 1's and 0's. Select lines 104 are similarly applied with high or low signals. The specific pattern of high or low signals at select lines 104 determines which of signal inputs 102 is connected to output 106. Once the connection is made between a signal input and the output, or, equivalently, a signal is selected and "passed" to the output, the passed signal is then available at the output after a short delay. Signal inputs are, typically, outputs from a memory element such as random access memory (RAM) or read-only memory (ROM).

As an example, assume that only select lines 108 and 112 are used. Further, assume that there are only four signal inputs 102. A common operation of the look-up table is then as follows: When select line 108 and select line 112 are both low, input 114 is passed to output 106. When select line 108 is low and select line 112 is high, input 116 is passed to output 106. When select line 108 is high and select line 112 is low, input 118 is passed to output 106. Finally, when select line 108 and select line 112 are both high, signal input 120 is passed to output 106. Thus, by using a binary numbering scheme at the select lines 104, signal inputs are passed to the output according to their number corresponding with their position from top to bottom in FIG. 1.

FIG. 2 is a schematic diagram of a prior art look-up table circuit. In FIG. 2, look-up table circuit 150 includes select lines A, B, C and D at 152, signal inputs R0-15 at 154 and outputs, LOUT, at 156.

In the circuit of FIG. 2, voltages in the form of digital signals are applied to signal inputs 154 and select lines 152. For example, in a typical implementation, the voltages may correspond to 0 volts for a "low" and 5 volts for a "high." With a low voltage corresponding to a "0" binary digit and a high voltage corresponding to a "1" binary digit there are 16 possible combinations of voltages that can be applied to select lines 152 to select one of the 16 signal inputs 154 as shown in Table I below.

                  TABLE I                                                          ______________________________________                                                 DCBA         LOUT                                                      ______________________________________                                                 1111         R0                                                                1110         R1                                                                1101         R2                                                                1100         R3                                                                1011         R4                                                                1010         R5                                                                1001         R6                                                                1000         R7                                                                0111         R8                                                                0110         R9                                                                0101         R10                                                               0100         R11                                                               0011         R12                                                               0010         R13                                                               0001         R14                                                               0000         R15                                                       ______________________________________                                    

As shown in Table I, where select lines A, B, C and D are each supplied with a high voltage, the signal input R0 is the signal seen at the output 156, LOUT. This can be verified by tracing signal input R0 to transistor 158. Since select line A is high, transistor 158 will be on and signal input R0 will be passed to transistor 160. Since select line B is high, signal input R0 will further be passed to transistor 162. Likewise, select lines C and D are high so that transistors 162, 164 will pass signal input R0 to LOUT at 156.

The transistors are grouped into four stages corresponding to the order in which a signal passes through the transistors. First stage 176 includes transistor 158, second stage 178 includes transistor 160, third stage 180 includes transistor 162 and fourth stage 182 includes transistor 164.

In the circuit of FIG. 2, each of the transistors 152, 160, 162 and 164 that passes signal input R0 to LOUT introduces a delay referred to as a "transistor delay." In a typical metal-oxide-semiconductor ("MOS") implementation, a single transistor delay is about 0.3 nS. Thus, the total propagation delay through the circuit of FIG. 2 is 1.2 nS.

A second example to illustrate the performance of the look-up table circuit of FIG. 2 assumes that the select lines have the value "1110" so that select lines D, C and B are high while select line A is low. This means that signal input R1 will be passed to LOUT. However, in order for signal input R1 to be passed to LOUT, transistor 174 must be on. Since select line A is low at the input to inverter 172 the gate of transistor 174 will be high so that transistor 174 is on.

An MOS inverter such as inverter 166, 168, 170 or 172 each has an inverter delay of about 0.5 Ns. In the cases where an inverted select line signal is used to pass a signal input, the delay of the inverter must be taken into account. Thus, where signal input R1 is passed through transistor 174 by enabling the gate of transistor 174 with the output of inverter 172, the delay in the first stage is the time required to turn transistor 174 on (0.5 nS) plus the transistor delay time in passing the signal from the source to the drain (0.3 nS). This gives a total time through a first stage transistor with an inverted select line signal at its gate of 0.8 nS.

The inverter delay of 0.5 nS is not a factor in later stages 2, 3 or 4 since after the first 0.5 nS of operation of the circuit the output of each inverter is available. This is because the select line signals propagate through the inverters "concurrently" or "in parallel." Thus, later stages only introduce a single transistor delay of 0.3 Ns. This means that the worst case total delay in passing a signal input to the output is about 1.7 nS. The general "best case" delay, for purposes of this discussion, is about 1.2 nS which occurs when a signal is passed through transistors that do not have an inverted select line controlling the transistor gate in the first stage. (A "special best case" delay not considered in this discussion occurs when select line D goes from low to high while select lines A, B and C do not change. This introduces only a 0.3 nS delay through transistor 164 before the signal at output 156 is valid.)

Thus, it is seen in the prior art circuit of FIG. 2 that the delay in selecting a signal and making the signal available at the output, LOUT, is in the range of 1.2 nS to 1.7 nS. Naturally, it is desirable to reduce this delay as much as possible.

SUMMARY OF THE INVENTION

The present invention is a look-up table circuit that provides for a reduced delay time in selecting and passing a signal input to the output. The invention includes combinational means such as NOR gates to derive signals based on two or more select lines. The output of the combinational means is used to enable transistors to pass the selected signal to the output.

A preferred embodiment implements a 16 input look-up table by having two of the four select lines used to derive inputs to four NOR gates. The resulting circuit requires only three transistor stages and thus reduces the delay time over prior art circuits such as the circuit of FIG. 2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a look-up table showing signal inputs, select lines and an output;

FIG. 2 is a prior art look-up table circuit;

FIG. 3 shows a look-up table circuit of the present invention having two transistor stages; and

FIG. 4 is a look-up table circuit of the present invention having three transistor stages.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 shows a first implementation of a look-up table circuit of the present invention. Note that select lines 302 labeled A, B, C and D are inputs to combinational subcircuits 304 and 306 and there are only two transistor stages 308 and 310. Subcircuits 304 and 306 output enable signals coupled to the gates of first and second stage transistors to turn these transistors on or off. The circuit of FIG. 3 performs according to Table I, above, as discussed in connection with FIG. 2.

To illustrate the performance of FIG. 3 assume that select lines A, B, C and D are all high. NOR gates such as 320 of subcircuit 304 will output a high signal only when both of the inputs to the NOR gate are low. Since select lines A and B are both high, it is shown from the schematic of subcircuit 304 in FIG. 3 that only NOR gate 320 will output a high signal since each of the other NOR gates in subcircuit 304 has as an input either uninverted select line A or uninverted select line B. Thus, four of the signal inputs, namely, R0, R4, R8 and R12 are passed through first stage 308 of FIG. 3.

Similar to subcircuit 304, subcircuit 306 includes four NOR gates whose inputs are connected to signals derived from the select lines. In subcircuit 306, since select lines C and D are both high, NOR gate 322 is the only NOR gate outputing a high signal. This means that only transistor 324 will have a high voltage at its gate and will pass its signal to LOUT. This passed signal is R0.

It is similarly shown that other combinations of high and low voltages on select lines A, B, C and D will result in a different signal input being passed to the output in accordance with Table I above.

Note that the total delay of the circuit of FIG. 3 differs from the prior art circuit of FIG. 2. Each signal input 303 only passes through two transistor stages before reaching the output, LOUT, and, hence, there are only two transistor delays. However, there is a longer delay in obtaining enable signals from the subcircuits 304 and 306 to turn the transistors on. In the worst case an enable signal such as enable signal 305 requires an inverter gate delay and a NOR gate delay before enable signal 305 is valid. As discussed above, an inverter delay is approximately 0.5 nS. A NOR gate delay is also approximately 0.5 nS. This means that the worst case delay for a valid enable signal, such as enable signal 305, is 1.0 nS. The worst case delay for a signal to propagate through first stage 308 is, therefore, about 1.3 nS.

The propagation delay through second stage 310 of FIG. 3 will be only one transistor delay since, for example, by the time the first stage delay output is valid at transistor 324 the signal at the gate of transistor 324 is also valid since the combinational means implemented by subcircuit 306 operates in parallel with subcircuit 304. This means that there is no delay introduced into the second stage as a result of waiting for signals at the gates of the transistors in second stage 310 to become valid. The delay added by second stage 310 is about 0.3 nS giving a total worst case delay time of 1.6 nS.

The best case delay through the first stage occurs when select lines A and B are both low so that the controlling enable signal is generated by NOR gate 321 which doesn't use outputs from inverters 312 or 314. In this case, the delay through first stage 308 is 0.8 nS. Through the second stage, the best case delay is 1.1 nS which assumes the case where the enable signal from subcircuit 306 is not generated from a NOR gate with inputs coupled to the inverted select lines, i.e., select lines C and D are both low. In computing the second stage delay, note that the delay through NOR gate 323 occurs in parallel with the delay through NOR gate 321 and, hence, does not contribute to the total delay.

It is thus shown that, in the look-up table circuit of FIG. 3, the total delay in passing the signal input to the output is in the range 1.1 nS to 1.6 nS. This is the better than the prior art circuit of FIG. 2.

FIG. 4 shows look-up table circuit 400 that is another, preferred, embodiment of the present invention. The embodiment of FIG. 4 only uses one subcircuit 402 instead of the two subcircuits used in the circuit of FIG. 3 above. As before, the look-up table circuit of FIG. 4 includes select lines at 404, signal inputs at 406 and an output at 408. The circuit of FIG. 4 has three transistor stages, first stage 410, second stage 412 and third stage 414. The circuit of FIG. 4 operates in accordance with Table I above.

To illustrate the performance of FIG. 4, assume that select lines A and B are high. This results in signal inputs R0, R2, R4, R6, R8, R10, R12 and R14 in being passed through first stage 410. The time required for the signals to propagate through the first stage is one transistor delay, or 0.3 nS.

Second stage 412 introduces, at most, another 0.3 nS single transistor delay since the inverted select line B enabling signals will already be present at stage two having been concurrently derived with select line A signals. In the example, signals R0, R4, R8 and R12 are passed through the second stage with a delay of 0.6 nS.

At third stage 414, the enable signals to turn on transistors 422, 424, 426 and 428 are the outputs of subcircuit 402. Subcircuit 402, like subcircuits 304 and 306, is a combinational circuit using inverters and NOR gates to generate enable signals for transistors in stage three at 414. The minimum delay for valid enable signals is when select lines C and D are both low. This results in a high signal at the output of NOR gate 425 after 0.5 nS delay. Since this 0.5 nS delay is concurrent with the delay through the first and second stages, the delay attributable to NOR gate 425 is inconsequential. The high signal output from NOR gate 425 is applied to the gate of transistor 428 to select R12, in the present example, after a transistor delay of 0.3 nS. Thus, the best case delay, which occurs when select lines C and D are low and select lines A and B are high, is 0.9 nS for the circuit of FIG. 4.

To analyze the worst case delay for the circuit of FIG. 4, assume that select line A is low. Then inverter 420 causes a delay of about 0.5 nS resulting in a 0.8 nS worst case delay through the first stage. The worst case propagation delay in combined stages one and two is therefore 1.1 nS. The maximum delay for valid enable signals appearing at the outputs of subcircuit 402 is 1.0 nS, when both select lines C and D are high. However, since the delay from stages one and two is 1.1 nS in the worst case, the delay attributable to subcircuit 402 does not affect the worst case total delay. In other words, the outputs of subcircuit 402 will be valid by the time the selected signal input has propagated to the third stage transistors at 414. Thus, stage three at 414 will add 0.3 nS delay for a total of 1.4 nS total delay time in the worst case. This means that with all select lines high, the time to propagate R0 to LOUT is 1.4 nS.

Thus, the circuit of FIG. 4 provides a look-up table circuit with delay times in the range of 0.9 nS to 1.4 nS. This is a marked improvement over the prior art.

Note that other combinational means may be used in place of subcircuit 402 of FIG. 4 and subcircuits 304 and 306 of FIG. 3. For example, NOR gate 424 can be substituted with an AND gate with inputs connected directly to select lines C and D without going through inverters 430 and 432. However, inverters and NOR gates are the preferred devices since they are easier and more economical to fabricate on a semiconductor chip.

In the foregoing specification, the invention has been described with reference to a specific, exemplary embodiment thereof. It will, however, be evident that various modifications and changes may be made there onto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The figures in this specification apply for a typical CMOS process and will be different depending on the technology used to fabricate the look-up table device. Other transistor technologies such as bipolar may be used to implement the circuits of the present invention. Further, the circuit may be operated at voltages other than those specifically disclosed herein without departing from the scope of the invention. The circuit may also be used to implement a "multiplexer" function. Many such changes or modifications will be readily apparent to one of ordinary skill in the art.

The specifications and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense, the invention being limited only by the provided claims. 

What is claimed is:
 1. A look-up table circuit for an integrated circuit comprising:a first level of pass transistors comprising:a first pass transistor; a second pass transistor; a third pass transistor; and a fourth pass transistor, wherein the first pass transistor, second pass transistor, third pass transistor, and fourth pass transistor are coupled, in parallel, to a look-up table output node; a second level of pass transistors comprising:a first pass transistor; a second pass transistor; a third pass transistor; and a fourth pass transistor, wherein the first pass transistor, second pass transistor, third pass transistor, and fourth pass transistor of the second level of pass transistor are coupled, in parallel, to one transistor in the first level of pass transistors; and a plurality of NOR gates, each coupling to a different pass transistor of the first and second levels of pass transistors.
 2. A programmable logic device with a look-up table circuit as recited in claim
 1. 3. A look-up table circuit for an integrated circuit comprising:a first level of pass transistors comprising:a first pass transistor; a second pass transistor; a third pass transistor; and a fourth pass transistor, wherein the first pass transistor, second pass transistor, third pass transistor, and fourth pass transistor are coupled, in parallel, to a look-up table output node; a second level of pass transistors comprising:a first pass transistor; a second pass transistor; a third pass transistor; and a fourth pass transistor, wherein the first pass transistor, second pass transistor, third pass transistor, and fourth pass transistor of the second level of pass transistor are coupled, in parallel, to one transistor in the first level of pass transistors; a first decoding block comprising:a first NOR gate coupled to the first pass transistor of the first level of pass transistors; a second NOR gate coupled to the second pass transistor of the first level of pass transistors; a third NOR gate coupled to the third pass transistor of the first level of pass transistors; and a fourth NOR gate coupled to the fourth pass transistor of the first level of pass transistors; and a second decoding block comprising:a first NOR gate coupled to the first pass transistor of the second level of pass transistors; a second NOR gate coupled to the second pass transistor of the second level of pass transistors; a third NOR gate coupled to the third pass transistor of the second level of pass transistors; and a fourth NOR gate coupled to the fourth pass transistor of the second level of pass transistors.
 4. A look-up table circuit for an integrated circuit comprising:a first level of pass transistors comprising:a first pass transistor; a second pass transistor; a third pass transistor; and a fourth pass transistor, wherein the first pass transistor, second pass transistor, third pass transistor, and fourth pass transistor of the second level of pass transistor are coupled, in parallel, to a look-up table output node; a second level of pass transistors comprising:a first pass transistor; a second pass transistor; a third pass transistor; and a fourth pass transistor, wherein the first pass transistor, second pass transistor, third pass transistor, and fourth pass transistor of the second level of pass transistors are coupled, in parallel, to one transistor in the first level of pass transistors; a first decoding block comprising:a first NOR gate coupled to the first pass transistor of the first level of pass transistors; a second NOR gate coupled to the second pass transistor of the first level of pass transistors; a third NOR gate coupled to the third pass transistor of the first level of pass transistors; and a fourth NOR gate coupled to the fourth pass transistor of the first level of pass transistors; a second decoding block comprising:a first NOR gate coupled to the first pass transistor of the second level of pass transistors; a second NOR gate coupled to the second pass transistor of the second level of pass transistors; a third NOR gate coupled to the third pass transistor of the second level of pass transistors; and a fourth NOR gate coupled to the fourth pass transistor of the second level of pass transistors; a first select signal to the look-up table circuit coupled to the third and fourth NOR gates of the second decoding block; a complementary signal of the first select signal coupled to the first and second NOR gates of the second decoding block; a second select signal to the look-up table circuit coupled to the second and third NOR gates of the second decoding block; a complementary signal of the second select signal coupled to the first and fourth NOR gates of the second decoding block; a third select signal to the look-up table circuit coupled to the second and fourth NOR gates of the first decoding block; a complementary signal of the third select signal coupled to the first and third NOR gates of the first decoding block; a fourth select signal to the look-up table circuit coupled to the third and fourth NOR gates of the first decoding block; and a complementary signal of the fourth select signal coupled to the first and second NOR gates of the first decoding block.
 5. A look-up table circuit for an integrated circuit comprising:a first stage of pass transistors coupling to an output of the look-up table circuit; a second stage of pass transistors; comprising:a first group of transistors coupling one of a plurality a signal inputs to a first pass transistor of the first stage; a second group of transistors coupling one of the plurality a signal inputs to a second pass transistor of the first stage; a third group of transistors coupling one of the plurality a signal inputs to a third pass transistor of the first stage; and a fourth group of transistors coupling one of the plurality a signal inputs to a fourth pass transistor of the first stage; a first decoding block coupling to the first stage comprising:a first NOR gate coupled to the first pass transistor of the first stage; a second NOR gate coupled to the second pass transistor of the first stage; a third NOR gate coupled to the third pass transistor of the first stage; and a fourth NOR gate coupled to the fourth pass transistor of the first stage; a first select signal coupled to the second and fourth NOR gates of the first decoding block; a complementary signal of the first select signal coupled to the first and third NOR gates of the first decoding block; a second select signal coupled to the third and fourth NOR gates of the first decoding block; and a complementary signal of the second select signal coupled to the first and second NOR gates of the first decoding block; a second decoding block, coupling to the second stage of pass transistors, comprising:a first NOR gate coupled to a first pass transistor in each group of the second stage; a second NOR gate coupled to a second pass transistor in each group of the second stage; a third NOR gate coupled to a third pass transistor in each group of the second stage; and a fourth NOR gate coupled to a fourth pass transistor in each group of the second stage; a third select signal coupled to a third -NOR gate and a fourth NOR gate of the second decoding block; a complementary signal of the third select signal coupled to a first NOR gate and a second NOR gate of the second decoding block; a fourth select signal to the look-up table circuit coupled to a second and third NOR gates of the second decoding block; and a complementary signal of the fourth select signal coupled to the first and fourth NOR gates-of the second decoding block.
 6. A look up table circuit, including:a first stage including:a first sub-stage including a first pass transistor, a second pass transistor, a third pass transistor, and a fourth pass transistor coupled between a first node and a first signal input, a second signal input, a third signal input, and a fourth signal input respectively; a second sub-stage including a fifth pass transistor, a sixth pass transistor, a seventh pass transistor, and an eighth pass transistor coupled between a second node and a fifth signal input, a sixth signal input, a seventh signal input, and an eighth signal input respectively; a third sub-stage including a ninth pass transistor, a tenth pass transistor, an eleventh pass transistor, and a twelfth pass transistor coupled between a third node and a ninth signal input, a tenth signal input, an eleventh signal input, and a twelfth signal input respectively; and a fourth sub-stage including a thirteenth pass transistor, a fourteenth pass transistor, a fifteenth pass transistor, and a sixteenth pass transistor coupled between a fourth node and a thirteenth signal input, a fourteenth signal input, a fifteenth signal input, and a sixteenth signal input respectively; a first select circuit coupled to receive a first select signal and a second select signal, and depending on the state of the first select signal and the second select signal, the first select circuit configured to enable one of four sets of the pass transistors of the first stage:the first set including the first pass transistor, the fifth pass transistor, the ninth pass transistor, and the thirteenth pass transistor, the first set configured to selectively couple the first, fifth, ninth, and thirteenth signal inputs to the first, second, third, and fourth nodes of the first stage respectively in response to a first state of the first and second select signals; the second set including the second pass transistor, the sixth pass transistor, the tenth pass transistor, and the fourteenth pass transistor, the second set configured to selectively couple the second, sixth, tenth, and fourteenth signal inputs to the first, second, third, and fourth nodes of the first stage respectively in response to a second state of the first and second select signals; the third set including the third pass transistor, the seventh pass transistor, the eleventh pass transistor, and the fifteenth pass transistor, the third set configured to selectively couple the third, seventh, eleventh, and fifteenth signal inputs to the first, second, third, and fourth nodes of the first stage respectively in response to a third state of the first and second select signals; and the fourth set including the fourth pass transistor, the eighth pass transistor, the twelfth pass transistor, and the sixteenth pass transistor, the fourth set configured to selectively couple the fourth, eighth, twelfth, and the sixteenth signal inputs to the first, second, third, and fourth nodes of the first stage respectively in response to a fourth state of the first and second select signals; a second stage including a first pass element, a second pass element, a third pass element, and a fourth pass element selectively coupled between an output node and the first node, the second node, the third node, and the fourth node of the first stage; and a second select circuit coupled to receive a third select signal and a fourth select signal, and depending on the state of the third select signal and the fourth select signal, the second select circuit configured to enable a selected one of the first pass element, the second pass element, the third pass element or the fourth pass element to selectively couple the output node to one of the first, second, third, or fourth nodes of the first stage, wherein a data signal at a selected one of the sixteen signal inputs, as determined by the state of the four select signals, can propagate through the first and second stages to the output node of the look up table circuit.
 7. The look up table circuit of claim 6, wherein the first select circuit includes four logic elements configured to generate four enable signals respectively.
 8. The look up table circuit of claim 7, wherein the four enable signals enable the pass transistors of the first set, the second set, the third set, and the fourth set respectively.
 9. The look up table circuit of claim 7, wherein the four logic elements are four NOR gates respectively, and each of the four NOR gates receives one of four possible logical combinations of the first select signal and the second select signal such that only one of the four enable signals is enabled at a time.
 10. The look up table circuit of claim 6, wherein the second selection circuit is a logic circuit including four logic elements configured to generate four enable signals respectively.
 11. The look up table circuit of claim 10, wherein the four enable signals enable the first pass element, the second pass element, the third pass element, and the fourth pass element respectively.
 12. The look up table circuit of claim 10, wherein the four logic elements are four NOR gates respectively, and each of the four NOR gates receives one of four possible logical combinations of the third select signal and the fourth select signal such that only one of the four enable signals is enabled at a time.
 13. The look up table circuit of claim 6, wherein sixteen data signals are applied to the sixteen signal inputs of the first stage.
 14. The look up table circuit of claim 6, wherein sixteen stored data signals are applied to the sixteen signal inputs of the first stage.
 15. The look up table circuit of claim 6, wherein the first pass element, the second pass element, the third pass element, and the fourth pass element are each transistors.
 16. A look up table circuit, including:a first stage including a set of N input nodes and a sub-set of intermediate nodes, the first stage configured to receive input signals at the N input nodes respectively and to pass a selected sub-set of the input signals to the intermediate nodes, wherein the first stage includes a set of N pass transistors, each one of the N pass transistors being coupled between a selected one of the intermediate nodes and a selected one of the N input nodes respectively; a first decoder, coupled to the first stage, and configured to select the sub-set of input signals in response to the state of a first group of select signals, the first decoder further configured to provide first enable signals to the first stage to enable the first stage to pass the selected sub-set of input signals to the intermediate nodes, wherein the first decoder activates a different one of the first enable signals in response to each possible combination of the first group of select signals; a second stage, coupled between the intermediate nodes of the first stage and an output node of the look up table circuit, the second stage configured to receive the selected subset of the input signals at the intermediate nodes and to pass a selected one input signal among the sub-set of input signals to the output node; and a second decoder, coupled to the second stage, and configured to select the selected one input signal among the subset of input signals in response to the state of a second group of select signals, the second decoder further configured to provide second enable signals to the second stage to enable the second stage to pass the selected one input signal to the output node.
 17. The look up table circuit of claim 16, wherein each of the first enable signals is coupled to activate a different sub-set of pass transistors among the set of N pass transistors such that the selected sub-set of input signals passed to the intermediate nodes is different with the activation of each one of the first enable signals.
 18. A look up table circuit including:a first stage including a set of N input nodes and a sub-set of intermediate nodes, the first stage configured to receive input signals at the N input nodes respectively and to pass a selected sub-set of the input signals to the intermediate nodes; a first decoder, coupled to the first stage, and configured to select the sub-set of input signals in response to the state of a first group of select signals, the first decoder further configured to provide first enable signals to the first stage to enable the first stage to pass the selected sub-set of input signals to the intermediate nodes, wherein the first decoder includes a plurality of logic gates coupled to receive all the possible combinations of the first group of select signals and their complements respectively; a second stage, coupled between the intermediate nodes of the first stage and an output node of the look up table circuit, the second stage configured to receive the selected sub-set of the input signals at the intermediate nodes and to pass a selected one input signal among the sub-set of input signals to the output node; and a second decoder, coupled to the second stage, and configured to select the selected one input signal among the sub-set of input signals in response to the state of a second group of select signals, the second decoder further configured to provide second enable signals to the second stage to enable the second stage to pass the selected one input signal to the output node. 